2‐Phosphanylphenolate Nickel Catalysts for the Polymerization of Ethylene

Abstract

The previously unknown methallylnickel 2-diorganophosphanylphenolates (R=Ph, cHex) were synthesized and found to catalyze the polymerization of ethylene. To explore the potential for ligand-tuning, a variety of P-alkyl- and P-phenyl-2-phosphanylphenols was synthesized and allowed to react with [Ni(cod)(2)] (cod=1,5-cyclooctadiene) or with NiBr(2).DME and NaH. The complexes formed in situ with [Ni(cod)(2)] are generally active as ethylene polymerization catalysts with all the ligands tested, whereas the latter systems are inactive when 2-dialkylphosphanylphenols are applied. M(w) values, ranging from about 1000 to about 100000 g mol(-1), increase for various R(2)P groups in the order R=Ph<Et approximately tBu<iPr<cHex, while aryloxy substituents usually diminish the molecular weights. Increasing basicity of P- or aryloxy substituents usually leads to increasing turnover numbers. The polymers are high density polyethylene (HDPE) with low methyl and sometimes low long-chain branching. Investigation of the influence of the solvent shows that the catalysts are stabilized by some oxygen-containing Lewis bases as well as by olefins. The catalysts are tolerant to water, in particular the 2-dicyclohexylphosphanylphenolate nickel catalyst, which in THF-water (1:1) gives HDPE with M(w)=39000, M(n)=14500 g mol(-1). Additives of tetrahydrothiophene or pyridine do not inhibit the catalyst, but decrease the reaction rate and the molecular weight of the polymers.